Earth’s magnetic field—useful not just for navigation but for shielding the planet’s surface from the charged particles of the solar wind—owes its existence to the convective flow of the outer core. Heated from below and cooled from above, the churning liquid iron–nickel alloy hosts a self-sustaining dynamo in which electric currents and magnetic fields continually induce one another.
But the full picture is not so simple. The flow in Earth’s core is almost certainly turbulent—and therefore chaotic, nonlinear, and hard to model. Observing the flow field directly is impossible. Even studying turbulent flows in liquid metals in the lab is extremely challenging: Liquid metals are opaque, so the optical methods used to study turbulence in air, water, glycerol, and other fluids are inapplicable.
And there are good reasons to think that turbulence in liquid metals is fundamentally different from turbulence in other fluids. Convective turbulence involves the interplay between the fluid’s velocity field and its temperature field, so it’s governed by the competition between the shear gradients and the thermal gradients that the fluid can sustain over space and time. In liquid metals, because of the high thermal conductivity, the shear gradients win. For other fluids, it’s a much closer race.
In the magnetohydrodynamics department at the Helmholtz-Zentrum Dresden-Rossendorf in Germany, Sven Eckert leads a team of some 40 researchers in developing, refining, and applying techniques for measuring velocity fields in liquid metals. Light may not penetrate the liquids, but sound does, so one of the team’s go-to methods is ultrasound Doppler velocimetry (UDV): seeding a liquid metal with tiny acoustically reflective particles, and then using ultrasound transducers to measure their velocity profile along a set of one-dimensional beams through the liquid. Technical limitations make it hard to deploy more than about 20 transducers simultaneously, so the technique doesn’t give the full 3D velocity field. But it offers valuable clues about flow patterns nonetheless.
Now Eckert and colleagues, including PhD student Felix Schindler and project leader Tobias Vogt, have used UDV to study convective turbulent flow in a 64-cm-tall drum (shown in the left-hand figure) filled with a gallium-indium-tin mixture that’s liquid at room temperature. From their measurements (including the snapshot shown at right), they found a surprising result: The flow structure was constantly changing, and large-scale circulation was absent.
Large-scale circulation—one or a few big, swirling flow cells that fill the entire container and persist over time—is a stable and ubiquitous feature of even the most turbulent air and water convections. No theory had predicted that the situation would be any different for liquid metals.
Despite metals’ high thermal conductivity, the vast majority of heat transfer through a churning liquid metal occurs through convection—and that’s still true even when large-scale circulation shuts down. But the absence of large-scale circulation makes the total heat flow through the drum much lower than expected.
What does that mean for Earth’s core and magnetic field? It’s not yet clear. The strength of the heat flow driving a fluid’s turbulence is proportional to the temperature difference across the fluid times the cube of the fluid’s height. For Earth’s 2200-km-thick outer core, that parameter is orders of magnitude greater than for any lab experiment—and the researchers’ 64 cm drum was already veering into uncharted territory for liquid metals. But the researchers plan to explore different metals and different geometries, in the hope of establishing the foundations of a robust theory that can be extrapolated to planetary scales. (F. Schindler et al., Phys. Rev. Lett. 128, 164501, 2022.)
Editor’s note, 20 May: The article has been updated with the addition of the top photo and caption.
